In most people (80%), infection causes no symptoms.
In others, the virus causes mild flu-like symptoms known as West Nile fever. The virus is able to pass the blood-brain barrier, and the most serious effects (in 0.7% of the infected) are encephalitis (inflammation of the brain) and meningitis (inflammation of the lining of the brain and spinal cord), both of which can be fatal.
Persons over 50 years of age are at higher risk of developing severe disease, the symptoms of which include fever, nausea, stiff neck and changes in mental status.
In rare cases (first reported October 2002), patients develop sudden-onset paralysis.

Symptoms develop 3-14 days after infection.
No effective treatment is known.

The disease can be diagnosed by employing an ELISA test detecting IgM antibodies against the virus. However, several related viruses also cause encephalitis and result in similar antibodies.

The virus is mostly maintained in birds. Female mosquitos, mainly of the species Culex pipiens, Culex restuan and Culex quinquefasciatus, bite infected birds, carry the virus in their salivary glands and infect other birds when they bite again. It is unknown which mosquito species transmit the virus from birds to mammals. In mammals
the virus does not multiply as well and it is believed that mosquitos biting infected mammals do not further transmit the virus.

West Nile virus was first isolated from a feverish adult woman in the West Nile District of Uganda in 1937.
The ecology was characterized in Egypt in the 1950s.
The virus became recognized as a cause of severe human meningoencephalitis in elderly patients during an outbreak in Israel in 1957.
The disease was first noted in horses in Egypt and France in the early 1960s.

The first appearance of West Nile virus in North America in 1999, with encephalitis reported in humans and horses, and the subsequent spread in the United States, may be an important milestone in the evolving history of this virus.
Since the first North American cases in 1999, the virus has been reported throughout the United States and Canada east of the Rocky Mountains; there have been human cases and horse cases, and many birds--especially crows--are infected.

A very high level of media coverage through 2001/2002 raised public fears of West Nile virus, even though common diseases such as influenza take far more lives each year. This disproportionate coverage is most likely the result of the novelty of the disease and the successive announcements of the disease's initial appearance in new areas.

Environmentalists have condemned attempts to control the transmitting mosquitos by spraying pesticide, saying that the detrimental health effects of spraying outweigh the relatively few lives which may be saved, and that there are more environmentally friendly ways of controlling mosquitoes. There are also questions about the effectiveness of insecticide spraying, because mosquitos that are resting or flying above the level of spraying will not be killed; the commonest vector in the northeastern U.S., Culex pipiens[?], is a canopy[?] feeder.

Recent Outbreaks United States:
From 1999 through 2001, the CDC confirmed 149 cases of human West Nile virus infection, including 18 deaths.
In 2002 a total of 4156 cases were reported, including 284 fatalities. 13 cases in 2002 were contracted through blood transfusion. Through June 2003 there were no symptomatic human cases although one West Nile-infected blood transfusion was detected at the end of June.

Canada: Canada had found the virus in birds in Ontario, Nova Scotia, Manitoba, and Quebec in 2002. One human death occurred in 1999. Two human deaths out of 141 cases in 2002 were reported by Canadian health officials.